In the past it has been known to provide a time sharing laser system but it has remained highly desireable to provide a time sharing laser system which is arranged to share the laser beam in a manner so that each subsystem may use the full power in the beam. It is also required in such a time shared laser system that the system be able to switch the beam rapidly and efficiently between the two subsystems. In order to provide for coherent detection of the laser beam it is necessary to provide a local oscillator beam, having a controllable intensity, for each subsystem channel and to be able to mix the local oscillator beam efficiently with the signal beam, even when the signal beam returns off axis.
These needs prior to the present invention have heretofore been unmet. The U.S. Pat. No. 3,087,377 to W. J. Daley (issued Apr. 30, 1963) for a Polarized Light Autocollimator discloses an instrument for detecting very small changes in the angular rotation of a reflective element. The Daley system includes a beam splitter which reflects a portion of an incident beam, the transmitted portion being trapped and rejected, the reflected beam then being collimated and aimed at a reflecting element, the rotational displacement of which about an axis at right angles to a beam striking it is to be measured. The light reflected from the reflecting element passes through the beam splitter and then through a rotating quarter wave plate and is imaged by a split field analyzer and a lens on a photoelectric light detector. The light from the light source to the beam splitter is circularly polarized while the rotating quarter wave plate transforms the circularly polarized light into substantially plane polarized light, the plane of polarization of which rotates. It will be seen in the Daley system that it is merely an autocollimator for detecting very small changes in angular rotation of a reflective element and is not directed to nor discloses a time shared laser system which utilizes a polarizing beam splitter to switch the full power of the incident beam between two subsystems and to generate a local oscillator signal for coherent detection.
U.S. Pat. No. 3,956,626 to Monte Ross (issued on May 11, 1976) for a Pulse Quaternary Communication Means discloses a pulse quarternary modulation transmitter configuration which includes means for using a combination of polarization states and pulsed position information to represent information choices. The Ross system discloses first, second, and third modulator means, means associated with the first modulator means to predeterminately polarize the received optical pulses, and a polarizing beam splitter positioned to respond to the output of the first modulator means to separate the output pulses therefrom according to their polarization. The second and third modulator means then receive such separated pulse streams with appropriate delay means and synchronous polarizer means and drive means are provided to combine such outputs to produce a pulse train of optically modulated pulses for transmission. While disclosing a very sophisticated and complex laser system that does utilize a polarizing beam splitter, the Ross patent is basically concerned with providing a transmitter sending output laser pulses and such system is not directed to a time shared transceiver system showing the switching of the beam between the output ports nor does it disclose the generation of a local oscillator beam required for coherent detection.
U.S. Pat. No. 4,059,759 to Robert C. Harney et al (issued Nov. 22, 1977) and entitled Passive and Active Pulse Stacking Scheme for Pulse Shaping is directed to an apparatus and method for producing a sequence of radiation pulses with a pulse envelope of time variation which is controllable by an external electromagnetic signal applied to an active medium by a sectored reflector, through which the radiation passes. The Harney et al apparatus includes a magneto-optical modulator positioned intermediate a polarizer means and an analyzer means and further includes a power supply and optical switch means for externally producing a controllable, time-varying electromagnetic signal at the magneto-optical modulator in response to receipt of a portion of an incoming beam of light for producing a pulsed beam of light, a sequence of light pulses with a pulse envelope of controllable time variation. Although the Harney et al apparatus provides a beam splitter to direct a portion of a beam of pulsed electromagnetic radiation to the power supply and optical switch, it does not disclose a time shared transceiver laser system which utilizes a polarizing beam splitter to switch two signal beams with means provided to generate a local oscillator beam for each channel.
In the article entitled The Antenna Properties of Optical Heterodyne Receivers in the October 1966 issue, Vol. 5, No. 10 of APPLIED OPTICS, the need for a stable single-frequency local oscillator for optical heterodyne and homodyne receivers was recognized although no means for generating such a local oscillator was disclosed.